Automatic amplitude control for variable frequency oscillators



Patented July 29, 1947 AUTOMATIC AMPLITUDE CONTROL FOR VARIABLEFREQUENCY OSCILLATORS .Ranald D. Scheldorf, Haddon Heights, N. 5.,assignor to Radio Gorporation of America, a corporation oil DelawareApplication August ill, 1943, Serial No. 498,124

. 3 Claims. (Cl. 250-36) -means for connecting thereto differentinductive and capacitive elements, depending upon the operatingfrequency band, have been employed extensively in multiband radioreceivers and transmitters. One disadvantage of circuits of this type isthat the amplitude of oscillation varies considerably over the severalfrequency bands and over difierent portions of each band. Since it isoften desirable to maintain the oscillation amplitude at a substantiallyconstant value over the full range'of operating frequencies, it hasheretofore often been necessary to employ separate individually balancedoscillation circuits.

The instant invention contemplates a simple and efilcient circuit forcontrolling automatically the amplitude of oscillation provided by amultigrid oscillator tube. Briefly, the invention comprises a firsttriode interposed as a variable resister in the screen voltage supplycircuit connecting the anode and screen electrodes of a pentodeoscillator tube. A diode rectifier is connected to a suitable point inthe oscillation circuit to derive therefrom a positive control voltagecharacteristic of the amplitude of oscillations. This control voltage isapplied to the control electrode of a second triode which is directlycoupled to the control electrode of the first triode which functions asthe screen electrode resistor. The circuit effectively comprises atwo-stage amplifier for the control voltage which regulates theoperating voltage applied to the screen electrode oi the pentodeoscillator to maintain constant the regeneration, and hence theamplitude of oscillations generated thereby at a substantially constantvalue notwithstanding changes in the values of the oscillation circuitcomponents.

Among the objects of the invention are to provide an improved thermionictube oscillation circuit having automatic control of the amplitude ofoscillations generated therein. Another object of the invention is toprovide an improved automatic amplitude control circuit for a multibandthermionic tube oscillation circuit. An additional object of theinvention is to provide an improved automatic amplitude control circuitfor a multiband pentode oscillator. A further object of the invention isto provide an improved automatic amplitude control for a multigridoscillatortube wherein the operating voltage applied to the oscillatortube screen electrode is controlled by a thermionic tube circuitresponsive to the amplitude of oscillations generated by the oscillatortube.

The invention will be further described by reference to the accompanyingdrawing of which the single figure is a schematic circuit diagram of apreferred embodiment thereof.

Referring to the drawing, the oscillation circuit comprises a pentodeoscillator tube I havin its control electrode connected through aconventional parallelwise connected grid capacitor 2 and grid resistor 3to the movable element of a first wave-change switch 4. Similarly, thecathode of the pentode oscillator tube I is connected to the movableelement of a second wave-change switch 5. The movable elements of thefirst and second wave-change switches 4, 5, respectively, may be gangedas indicated by the dash line 6. The fixed contacts of the wave-changeswitches t and 5 are connected'to tapped points on an inductor i, oneend of which is connected to ground. Avariable tuning capacitor 8 isconnected between the movable element of the first wave-change switch tand the grounded end of the inductor l. The suppressor electrode of thepentode oscillator tube l is grounded. The pentode anode electrode isconnected to a source of positive potential, and also is connectedthrough a capacitor 9 to ground.

The anode of the pentode oscillator I is connected to the anode of afirst triode I Ii of a. double triode tube I i. The cathode of the firsttriode I0 is connected to the screen electrode of the pentode oscillatortube I, and also is connected through a fixed capacitor I2 to ground.-The anode of the second triode I3 of the double triode ll is connectedto the control electrode of the first triode l0, and also is connectedthrough a grid coupling resistor I4 to the cathode of the first triodeII). The cathode of the second triode I3 is connected through a cathoderesistor IE to ground, and also through a bleeder resistor I 6 to thescreen electrode of the pentode oscillator tube l.

The movable element of the first wave-change switch 4 is connected tothe anode of a diode rectifier tube I7. The cathode of the dioderectifier tube I1 is connected to the control electrode of the secondtriode I 3 of the double triode tube II, and is also connected through aconventional cathode resistance-capacitance network It to ground.

In operation, a screen voltage to the screen electrode of the pentodeoscillator tube I is supplied through the variable resistance comprising\the anode-cathode circuit of the first triode it of b i I I r 9,494,908

the double triode ll. Anode voltage for the second triode llof thedouble triode tube ll is obin, and applied to the control electrode ofthefirst triode Ill to vary the cathodeanode resistance of the firsttriode. Variations in the cathode-anode resistance of the first triode lprovide variable screen voltage on the Dentode oscillator l, whichautomatically controls the amplitude of oscillations generated thereby.The triode portions l0, l3 respectively, 01' the double triode controltube ll eflectively comprise a twostage amplifier responsive to thecontrol voltage derived from the diode rectifier I! for varying thescreen voltage on the pentode-oscillator in response to the amplitude ofoscillations generated thereby.

It will be seen that an increase in the amplitude of oscillationsgenerated ,by the pentode oscillator will provide increased positivebias on the control electrode of the second triode ll of the doubletriode control tube'l I, causing the second triode to draw more platecurrent and thereby biasing the first triode ill toward anode cur,-

rent cutofl. The effect of increasing the bias of the first triode l0toward anode current cutoff condition efiectively raises theanode-cathode resistance oi. the tube and lowers the screen voltageapplied to the .pentode oscillato i, which in turn, decreases theamplitude of oscillations generated in the oscillator tube circuit.

Furthermore, lowering oi! the voltage applied to the screen electrode ofthe pentode oscillator tube l lowers the fixed bias applied to thesecond triode cathode electrode, thereby increasing the apparent gain ofthe amplifier system.

Thus the invention described comprises a novel and efiicient means forcontrolling automatically the amplitude of oscillations of a multigridthermionic tube multiband oscillator circuit whereby control voltagesderived from the oscillation circuit provide efiective control of thescreen voltage of the oscillator tube.

I claim as my invention:

1. An automatic amplitude control circuit or a variable frequencyoscillator employing a thermionic discharge tube having at least acathode. an anode, a screen electrode and a control grid comprisingmeans including a second thermionic discharge tube providing a variableimpedance path from said anode to said screen electrode, rectifier meanscoupled to said oscillator tube control grid and responsive to theamplitude of oscillations generated by said oscillator for derivinlcontrol voltages, and means torapplying said control voltages to varythe impedance of said variable impedance path to control regeneration insaid oscillator by varying the voltage on said screen electrode.

2. An'automatic amplitude control circuit for a variable frequencyoscillator employing a thermionic discharge tube having at least acathode, an anode, a screen electrode and a control grid 4 comprisingmeansincluding a second thermionic discharge tube providing a variableimpedance path fromv said anode to said screen electrode, rectifiermeans coupled to said oscillator tube control grid and responsive to theamplitude of oscillations generated by said oscillator for derivingcontrol voltages. thermionic tube means for amplifying said controlvoltages and means for applying said amplified control voltages to saidsecond thermionic tube to vary the impedance of said variable impedancepath to control regeneration in said oscillator by varying the voltageonsaid screen electrode.

3. An automatic amplitude control circuit for a variable frequencyoscillator employing a thermionic discharge tube having at least acathode, an anode, a screen electrode and a control electrode comprisinga source of operating voltage for said anode, means including a secondthermionic discharge tube having at least at cathode, an anode and acontrol electrode providing a variable impedance path for said anodevoltage to said screen electrode, diode rectifier means coupled to saidoscillator tube control electrode and responsive to the amplitude ofoscillations generated by said oscillator for deriving control voltages,means including a third thermionic tube for amplifying said controlvoltages, and a connection from said third tube to the control electrodeof said second tube for applying said amplified control voltages to varythe impedance of said variable impedance path to vary the voltageapplied to said screen electrode 01' said oscillator tube.

RANALD D. SCHELDORF.

REFERENCES CITED The following references are of record in the file orthis patent:

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